Hydrophilic adhesives, particularly hydrophilic pressure-sensitive adhesives (“PSAs”), are used in a wide variety of commercially significant products, including drug delivery systems, wound dressings, bioelectrodes, tooth-whitening systems, and the like. A general distinctive feature of hydrophilic PSAs is that they typically adhere to wet substrates, while conventional hydrophobic (rubber-based) PSAs typically lose their adhesive properties when moistened.
It is important to be able to modify the adhesive properties of a PSA according to intended use, as different applications can require very different adhesion profiles. For instance, the skin contact adhesive layer of a transdermal drug delivery system, or “patch,” should provide for immediate adhesion following application of the patch to the skin and continued adhesion during an extended drug delivery period. As another example, delivery systems for application to wet surfaces, e.g., the buccal mucosa or the teeth, do not need to adhere to dry surfaces but should become tacky when applied to a hydrated or moistened surface. In another application, adhesive compositions used in wound dressings must become substantially nontacky following absorption of wound exudates to avoid tissue damage upon removal.
A method has recently been developed for tailoring the adhesive properties of polymer compositions useful in a number of applications, including pharmaceutical and cosmetic products. The method is based on new insights into the molecular mechanisms underlying adhesive properties. See, for example, Feldstein et al. (1999) Polym. Mater. Sei. Eng., 81 :465-466; Feldstein et al., General approach to the molecular design of hydrophilic pressure-sensitive adhesives, Proceed. 25th Annual Meeting Adhesion Soc. and 2nd World Congress on Adhesion and Relative Phenomena, February 2002, Orlando, Fla., vol. 1 (Oral Presentations), p. 292-294; and Chalykh et al. (2002) J Adhesion 78(8):667-694. As discussed in the foregoing references, pressure-sensitive adhesion results from the coupling of two apparently incompatible types of molecular structures, and there is a fine balance between strong cohesive interaction energy and enhanced “free volume.”
That is, enhanced free volume in the molecular structure of a PSA polymer composition correlates with high tack exhibited at the macroscopic level and a liquid-like fluidity of the PSA material, which, in turn, allow for rapid formation of an adhesive bond. The “cohesive interaction energy” or “cohesion energy” defines the cohesive toughness of the PSA composition and provides the dissipation of detachment energy in the course of adhesive joint failure. Based on these findings, a general method for obtaining novel hydrophilic adhesives was developed and is described in U.S. Pat. No. 6,576,712 to Feldstein et al. In one embodiment, that method involves physically mixing a non-adhesive, hydrophilic, high molecular weight polymer with a relatively low molecular weight plasticizer capable of crosslinking the polymer via hydrogen bonding.
In PSAs, the molecular structures of the components dictate the cohesion energy and free volume, and thereby define the adhesive properties of the composition as a whole. For instance, in acrylic PSAs, strong cohesive interaction energy is a result of hydrophobic attraction between alkyl groups in side chains, whereas large free volume results from either electrostatic repulsion of negatively charged carboxyl groups or a significant number of isoalkyl radicals in the side chains. In synthetic rubbers, large free volume is obtained by adding high volume, low density tackifying resins. In hydrophilic adhesives, when a high molecular weight polyvinyl lactam, e.g., poly(N-vinyl-2-pyrrolidone) (“PVP”) or polyvinyl caprolactone (“PVCap”), is blended with a polyethylene glycol (“PEG”) oligomer, as described in U.S. Pat. No. 6,576,712, high cohesive strength results from the hydrogen bonding interaction between the oxo (═O) moieties of the pyrrolidone or caprolactone ring and the terminal hydroxyl groups of the PEG oligomer, while enhanced free volume is results from the spacing between polymer chains provided by the PEG bridges and the flexibility of the PEG oligomers.
Accordingly, the balance between cohesive energy and free volume, as described in the '712 patent, is in large part responsible for the adhesive properties of polymer materials. For instance, the ratio between cohesion energy and free volume dictates the glass transition temperature, Tg, and elastic modulus, E, of a polymeric material. That is, a composition with higher cohesion energy and lower free volume will have both a higher Tg and a higher E.
When dry, the adhesive compositions described in U.S. Pat. No. 6,576,712, e.g. blends of high molecular weight PVP and low molecular weight PEG, exhibit relatively low adhesion toward dry surfaces. Adhesion increases, however, when the surface of a substrate is moistened or the adhesive composition absorbs water. The maximum adhesion of the PVPPEG blends described in the '712 patent is observed when the adhesive contains 5-10 wt. % of absorbed water (i.e., when water represents about 5 wt. % to about 10 wt. % of the moistened adhesive composition). This is usually the case when the adhesive is exposed to an atmosphere having 50% relative humidity (rh). When in direct contact with water, the adhesive dissolves. Therefore, the compositions are not optimal in applications wherein an adhesive composition is likely to undergo a significant degree of hydration during use, absorbing on the order of 15 wt. % water or more.
Accordingly, there is a need in the art for water-insoluble adhesive compositions that adhere well to moist surfaces even after absorbing a significant amount of water.